By lineal foot, I assume that you mean linear foot, which is a measurement of length. One board foot is equivalent to a board that measures 1 foot long by 1 foot wide by 1 inch thick. One board foot is equivalent to 144 cubic inches.
So, for example, if you have a board that is 12 feet long by 4 inches thick by 2 inches deep, then that would be 144 inches by 4 inches by 2 inches or 1152 cubic inches total. To convert from cubic inches to board feet, divide by 144 cubic inches, and the result is that that board is 8 board feet.